U-Pb baddeleyite geochronology of Precambrian mafic dyke swarms and complexes in southern Africa - regional scale extensional events and the origin of the Bushveld Complex
Sammanfattning: Dolerite dykes are formed when iron- and magnesium-rich (mafic) mantle-derived magmas ascend through the lithosphere (upper part of the mantle) and crystallize as ‘hydro-fractures’ within the crust. Dykes may exist in great numbers to form dyke swarms (linear or radiating), which can be linked to time periods of continental break-up, or attempted break-up events, associated with voluminous volcanism. Continental mafic dyke swarms are produced when the lithosphere is subjected to fracturing and therefore make up important markers of regional extensional events. Large dyke swarms have been hypothesized to represent the plumbing system to Large Igneous Provinces (LIPs), which are anomalously large magmatic settings (exceeding 100 000 km2) that apart from the dyke swarms also consist of thick flood basaltic lava flows and large mafic-ultramafic complexes. The origin of LIPs is debated, with the classical explanation involving melting triggered by a rising mantle plume (a column of very hot material from the deep mantle). Particularly dyke swarms with radiating patterns are believed to form during mantle plume generated melting events. Most of the continental landmasses on Earth contain very old (Archean; 3.8–2.5 billion years old) and geologically stable interiors, called cratons, which also preserve the oldest dyke swarms. Southern Africa encompasses the Kaapvaal and the Zimbabwe cratons, which were glued together (amalgamated) along the Limpopo Mobile Belt. Together, these two ancient cratons record some of the most spectacular geological formations on Earth, and preserve numerous mafic magmatic units including the largest mafic-ultramafic layered intrusion in the world, the Bushveld Complex, as well as abundant undated dolerite dyke swarms of a range of trends. The goal of this work was to provide initial dating on these swarms and investigate their links with known major magmatic events. In particular, a key goal was to investigate whether any Bushveld-aged radiating dykes exist in the Kaapvaal Craton; in order to test for its mantle plume origin. Rocks can be directly dated by isotopic systems on different types of minerals. Datings performed in the framework of this thesis comprises exclusively the uranium- (U) lead (Pb) isotope system on the mineral baddeleyite (ZrO2) to determine the crystallization age of dolerite dykes and their plutonic equivalents in the deeper parts of the crust. In short, U-Pb dating of mafic rocks in the oldest parts of the Kaapvaal Craton has revealed three separate, long-lived magmatic events, which represent important stages during the evolution of the craton, as well as a new model for the origin of the Bushveld Complex of which no mafic dykes have been found. There is evidence of a 2990–2965 million years old (Ma) mafic dyke swarm and associated mafic complex that was intimately related to the rifting of the Pongola Basin – the world’s oldest continental rift and associated Nsuze lavas. There is also a radiating swarm of mafic dykes that can be connected to a mantle plume that impacted the Kaapvaal lithosphere at 2700–2660 Ma and injected massive amounts of mafic magma into the more than 200 km thick lithosphere, as well as erupting as a thick sequence of flood basalts. At depth, these mafic rocks were transformed (metamorphosed) into denser compositions (eclogite rocks) and exerted an enhanced weight to the lithosphere, which controlled the Transvaal Basin development at the surface. Two billion years ago, Zimbabwe and Kaapvaal collided to form a single landmass. This tectonic event may have triggered the delamination of the dense plume-derived mafic root and thereby allowed for a rapid upwelling and massive decompressional partial melting of hot mantle, which produced the voluminous melts that had to be fed into the Bushveld Complex. Lastly, a northeast-trending dyke swarm was formed at 1875–1835 Ma along the northern border of the Kaapvaal Craton, and partially cutting across the Bushveld Complex. Together with the 1885–1870 Ma Mashonaland sill province in the Zimbabwe Craton, these intrusions represent the earliest coeval mafic magmatism across both cratons and hence support the hypothesis of a ca. 2.0 Ga collision between these two ancient cratons. However, this interpretation still remains to be tested by paleomagnetic constraints.
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